An Investigation into Transport Protocols and Data Transport ...

8.2. Test Calibration 152
cwnd and ssthresh (packets)
500
400
300
200
100
cwnd
ssthresh
Actual average cwnd
0
0 100 200 300 400 500 600
Time (seconds)
Figure 8.3: H-TCP cwnd dynamics showing the mode switch from low-speed into
high-speed modes (250Mbit/sec, 162ms RTT, queue-size 20% BDP).
higher value of cwnd upon loss as the calculation of β is overestimated 2 .
However, the main cause of the high cwnd values is due to the random loss
nature of the dummynet packet drops; there are occasions where the value
of cwnd reaches the next ‘stage’ in the HSTCP algorithm where the increase
and decrease parameters are higher; making the cwnd algorithm grow more
aggressively, and fall back to a relatively higher value upon loss.
Figure 8.2(b) shows the cwnd trace of ScalableTCP at 0.5% loss rate.
Again, it was observed that the actual average cwnd value of 24.3 is much
larger than the predicted 16 packets.
It is worth noting that 16 packets
is actually the threshold where ScalableTCP starts to engage its high-speed
mode, and the effects of this can be clearly seen as almost none of the congestion
events result in the Standard TCP cwnd drop to a half. As the decrease
parameter β of 0.875 is also implemented in the same way as HSTCP, it was
observed that the decrease in cwnd upon loss is actually less than theory due
to the quantisation of the values of cwnd (especially at such low values of
cwnd).
Figure 8.1 clearly shows that H-TCP is able to achieve very high utilisation
across the range of loss environments. Figure 8.3 shows the cwnd trace
2 As it is implemented traditionally where cwnd ← cwnd − b × cwnd where b = 1 − β.